Resistant starch and resistant dextrins are glucose polymers and oligomers generally obtained by heat treatment under acid conditions of starches (pyrodextrination). Such treatment involves different chemical reactions, including hydrolysis, transglucosidation, and repolymerisation. These reactions lead to the formation of random 1→2, 1→3, 1→4, and 1→6 glucosidic linkages. Since the human intestinal enzyme system is not capable of breaking glucosidic linkages other than α-D-(1→4), and α-D-(1→6), the converted dextrins are partly or wholly indigestible and have properties similar to resistant starch. They are functional dietary fibres having beneficial effects, such as stabilising intestinal flora, blunting post-prandial glucose levels and contributing to vascular health. Such resistant dextrins do not fall within three categories (RS 1-RS 3) of resistant starches as originally classified by Englyst et al., Eur. J. Clin. Nutr., 1992, 46 (suppl.), S33-S50. Resistant dextrins of this type are generally grouped among the type 4 resistant starch (RS 4, chemically modified resistant starch), although they have not been treated with other chemicals than acid, contrary to other modified starch such as starch ester and the like.
During acid and heat treatment of the starch (pyrodextrination), the starch gets darker (brown) due to decomposition and side reactions during the process. The resulting impurities are difficult to remove due to the fact that they are partially incorporated in the pyrodextrin structure during the treatment. Therefore these starches need to be purified before use in foodstuff.
U.S. Pat. No. 5,264,568 discloses a tedious multistep process for purification and isolation of a pyrodextrin hydrolysate.
EP 1799719 discloses a process for producing resistant starch by acidifying starch at 3% moisture at pH 2.3 in the presence of 4.3% of ethanol followed by heating at 140° C. This treatment is reported to result in 57.7% resistant starch, which is about an optimum, since lower (0.1%) and higher (8.3%) levels of ethanol result in lower yields (53.6%, 55.6%) of resistant starch. The products of EP 1799719 have a whiteness of 65 (on a scale of 0 to 100).
Hence, there is a need for providing a simplified and cost-effective process that can provide dextrinised starch with less impurities, improved colour properties, thus avoiding the need for any subsequent, extensive purification steps to tackle colour issues.